1. Field of the Invention
This invention relates to offshore drilling and producton work and more particularly it concerns novel methods and apparatus for positioning a working barge, such as a drilling or production rig, above a sea surface.
2. Description of the Prior Art
It has previously been proposed to provide a support arrangement which will support a shallow water drilling or production barge above the surface of the water at a deep water location. One such proposed support arrangment is described in U.S. Pat. No. 3,007,316 to A. J. Higgins, Jr. According to the Higgins patent, a platform structure made up of a group of submersible barges is floated to a desired offshore location and then submerged to the sea bottom. A shallow water barge is floated into position between frames extending up from outer ones of the submerged barges. A central submerged barge is then made buoyant and rises, while guided along the frames, until it contacts the bottom of the barge and lifts it up out of the water.
Another proposed support arrangement is described in U.S. Pat. No. 4,040,265 to L. W. Hellerman et al. According to the Hellerman et al patent, a mobil offshore platform of negative buoyancy is carried on a floating vessel to a desired offshore location. The negative buoyancy platform has vertical elongated support legs and jacking mechanisms to lower the legs to the sea bottom and then jack the platform up off the floating vessel. The platform is then jacked down until it is submerged, whereupon a working barge is floated over it. Then the platform is jacked up again on the legs to raise the barge up out of the water.
A still further proposal for positioning a working barge is described in the publication Drilling Contractor, November 1981 at pages 57-61. According to that publication, a jackup barge is floated to a drill site and is jacked up out of the water and ballasted with water to force the legs into the ocean bottom. The barge is then deballasted and lowered below the water surface. A working barge or "barge-rig" is then towed into position over the submerged jackup barge and the working barge is then ballasted or flooded so that it rests on top of the jackup barge.
U.S. Pat. No. 2,942,425 to L. B. DeLong et al describes a "mobil dry dock", fitted with jackup legs, to lift marine craft up out of the water. The DeLong et al device is made up of an upper working platform and a lower marine craft receiving and supporting platform with elongated legs passing through each platform and jacking mechanisms on the lower platform. The lower platform comprises a flotatable hull capable of floating and supporting the upper platform and the jacking legs. The lower flotatable hull platform is also provided with a propulsion unit. When the DeLong et al device is floated into position the legs are lowered to the sea bottom and the platforms are jacked up above the water. The upper platform is then pinned to the legs and the lower platform is lowered until it becomes submerged. The patent states that valves are provided in the lower platform to permit water to enter the platform while it is being submerged. A marine craft to be raised is then floated over the submerged lower platform. The lower platform is then jacked up on the legs to raise the marine craft up out of the water.
The techniques proposed by Higgins, Hellerman et al and DeLong et al all require complex and expensive equipment. The techniques proposed by DeLong and in the Drilling Contractors publication suffer from the further disadvantage that the very platform on which the jacking mecahnisms are mounted must be submerged to permit the marine vessel or working barge to float over it. This poses substantial problems in mounting and arranging the jacks so that they can be operated efficiently and reliably. Also, in the case of the DeLong et al arrangement, the rate at which the lower platform can be submerged or raised out of the water depends on how rapidly the platform can be filled or drained through its valves.